Update minisymposia/workshops: Link proceedings and add/fix slides URLs

[MakisH]

Closes #541.

Updates URLs in:

• https://precice.org/eccomas-congress-2022.html (issues new Nextcloud URLs, not sure why the old ones did not work anymore)
• https://precice.org/eccomas-coupled-2021.html (replaces conference URL with web archive)
• https://precice.org/community-events-wccm-eccomas-2020.html (replaces conference URL with web archive, adds book of abstracts on HAL, adds proceedings on Scipedia).

Also adds links to the slides on Zenodo to the Workshop 2025 page. Recordings could follow once the playlist is complete.

Also converts the language from present to past for the past minisymposia.